W-4 Witholdings : My wife and I both have full time jobs. I just picked up a part time job. We have 1 child. How do I fill out my W-4 Withholding Certificate?

You can use the Tax Withholding Estimator on the IRS web site to figure out what you should each put on the W-4 forms that you give to your employers. It will let you adjust your withholding to get the approximate refund amount that you want.

Tax Withholding Estimator

The W-4 form is 4 pages with instructions and worksheets, not just the front page you turn in to your employer.  You will need to analyze your situation and then probably turn in new W-4 forms to all three jobs (especially if you have not used the new form at your other jobs yet.)

Either use the worksheets and instructions on all pages of the W-4 to account for your 3 jobs, your deductions, and any dependents; or use the IRS withholding calculator as mentioned in the other answer.